These are the main causes for this issue:
- High amount of people connecting to the site.
- The settings.php isn't configured correctly.
It's a common problem for people who have a host which simply can't keep up with the users connecting or because of a misconfiguration of the settings.php. However, I should imagine the forum is reaching it's max connections which is causing the problems because it's not happening constantly which would indicate to the settings being configured incorrectly.
Both are easily resolved by either upgrading the host or by using this documentation:
Here 
